Performance improvement for last commit cache and show-ref (#15455) (#15701)

Backport #15455

* Improve performance when there are multiple commits in the last commit cache

* read refs directly if we can

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/commit_info_nogogit.go

@@ -102,10 +102,13 @@ func (tes Entries) GetCommitsInfo(commit *Commit, treePath string, cache *LastCo
 }
 
 func getLastCommitForPathsByCache(commitID, treePath string, paths []string, cache *LastCommitCache) (map[string]*Commit, []string, error) {
+	wr, rd, cancel := CatFileBatch(cache.repo.Path)
+	defer cancel()
+
 	var unHitEntryPaths []string
 	var results = make(map[string]*Commit)
 	for _, p := range paths {
-		lastCommit, err := cache.Get(commitID, path.Join(treePath, p))
+		lastCommit, err := cache.Get(commitID, path.Join(treePath, p), wr, rd)
 		if err != nil {
 			return nil, nil, err
 		}

modules/git/last_commit_cache_nogogit.go

@@ -7,6 +7,8 @@
 package git
 
 import (
+	"bufio"
+	"io"
 	"path"
 )
 
@@ -34,7 +36,7 @@ func NewLastCommitCache(repoPath string, gitRepo *Repository, ttl func() int64,
 }
 
 // Get get the last commit information by commit id and entry path
-func (c *LastCommitCache) Get(ref, entryPath string) (interface{}, error) {
+func (c *LastCommitCache) Get(ref, entryPath string, wr *io.PipeWriter, rd *bufio.Reader) (interface{}, error) {
 	v := c.cache.Get(c.getCacheKey(c.repoPath, ref, entryPath))
 	if vs, ok := v.(string); ok {
 		log("LastCommitCache hit level 1: [%s:%s:%s]", ref, entryPath, vs)
@@ -46,7 +48,10 @@ func (c *LastCommitCache) Get(ref, entryPath string) (interface{}, error) {
 		if err != nil {
 			return nil, err
 		}
-		commit, err := c.repo.getCommit(id)
+		if _, err := wr.Write([]byte(vs + "\n")); err != nil {
+			return nil, err
+		}
+		commit, err := c.repo.getCommitFromBatchReader(rd, id)
 		if err != nil {
 			return nil, err
 		}

modules/git/repo_commit_nogogit.go

@@ -9,9 +9,10 @@ package git
 import (
 	"bufio"
 	"errors"
-	"fmt"
 	"io"
 	"io/ioutil"
+	"os"
+	"path/filepath"
 	"strings"
 )
 
@@ -34,6 +35,18 @@ func (repo *Repository) ResolveReference(name string) (string, error) {
 
 // GetRefCommitID returns the last commit ID string of given reference (branch or tag).
 func (repo *Repository) GetRefCommitID(name string) (string, error) {
+	if strings.HasPrefix(name, "refs/") {
+		// We're gonna try just reading the ref file as this is likely to be quicker than other options
+		fileInfo, err := os.Lstat(filepath.Join(repo.Path, name))
+		if err == nil && fileInfo.Mode().IsRegular() && fileInfo.Size() == 41 {
+			ref, err := ioutil.ReadFile(filepath.Join(repo.Path, name))
+
+			if err == nil && SHAPattern.Match(ref[:40]) && ref[40] == '\n' {
+				return string(ref[:40]), nil
+			}
+		}
+	}
+
 	stdout, err := NewCommand("show-ref", "--verify", "--hash", name).RunInDir(repo.Path)
 	if err != nil {
 		if strings.Contains(err.Error(), "not a valid ref") {
@@ -69,6 +82,11 @@ func (repo *Repository) getCommit(id SHA1) (*Commit, error) {
 	}()
 
 	bufReader := bufio.NewReader(stdoutReader)
+
+	return repo.getCommitFromBatchReader(bufReader, id)
+}
+
+func (repo *Repository) getCommitFromBatchReader(bufReader *bufio.Reader, id SHA1) (*Commit, error) {
 	_, typ, size, err := ReadBatchLine(bufReader)
 	if err != nil {
 		if errors.Is(err, io.EOF) {
@@ -106,7 +124,6 @@ func (repo *Repository) getCommit(id SHA1) (*Commit, error) {
 	case "commit":
 		return CommitFromReader(repo, id, io.LimitReader(bufReader, size))
 	default:
-		_ = stdoutReader.CloseWithError(fmt.Errorf("unknown typ: %s", typ))
 		log("Unknown typ: %s", typ)
 		return nil, ErrNotExist{
 			ID: id.String(),